1. Field of the Invention:
The present invention relates in general to electrical induction logging systems for determining the characteristics of subsurface formations and in particular to an electrical induction logging system which utilizes multiple frequencies to measure the conductivity of subsurface formations. Still more particularly, the present invention relates to a multiple frequency electrical induction logging system which utilizes a closed loop conversion circuit to eliminate errors resulting from gain or phase variations.
2. Description of the Prior Art:
The nature and characteristics of various subsurface formations penetrated by a borehole are important considerations in the oil and gas industry during the drilling of that borehole. The existence, depth, location, quantity and other parameters concerning the oil and gas trapped within formations must be determined. Various techniques have been employed in the past to determine this information regarding formations penetrated by a borehole. One technique commonly utilized is induction logging. Induction logging measures the resistivity (or conductivity) of a formation by first inducing eddy currents to flow within these formations in response to an alternating current transmitter signal and then measuring a phase component signal in a received signal which is generated by the presence of those eddy currents. Variations in the magnitude of the eddy currents in response to variations in formation conductivity are then reflected as variations in the receiver signal. Thus, in general, the magnitude of a phase component of the receiver Signal, that component in-phase with the transmitter signal, is indicative of the conductivity of a formation.
In theory, the electrical resistivity of a formation should be relatively high when that formation contains a high percentage of hydrocarbons due to the fact that hydrocarbons are a poor conductor of electricity. Where hydrocarbons are not present in the formations and the formations contain salt water, the electrical resistivity of a formation should be relatively low. Formation water, which is typically highly saline, is a relatively good conductor of electricity. Induction resistivity logging tools thus obtain information regarding the formations which may be interpreted to indicate the presence or absence of hydrocarbons.
U.S. Pat. Nos. 3,340,464, 3,147,429, 3,179,879, and 3,056,917 are illustrative of typical prior art well logging tools which utilize the basic principles of induction logging. In each of the tools disclosed within these patents, a signal generator operates to produce an alternating current transmitter signal which is applied to a transmitter coil. The current in the transmitter coil induces a magnetic field in the surrounding formations. This magnetic field, in turn, causes eddy currents to flow within the formations. Because of the presence of these formation currents, a magnetic field is coupled into a receiver coil, thereby generating a receiver signal. Those skilled in the art will appreciate that such logging tools typically include a receiver coil and a transmitter coil which may each be comprised of multiple coils arranged in a predetermined fashion to obtain a desired response. The receiver signal is typically then amplified and applied to one or more Phase Sensitive Detectors (PSD). Each Phase Sensitive Detector then detects a phase component signal having the same phase as a phase reference signal which is also applied to the detector. The phase reference signal has a predetermined phase relationship to the current in the transmitter coil and the output of the phase sensitive detector may be further processed downhole or may be sent to the surface for processing by surface equipment.
A quantative determination of the conductivity of formations surrounding a borehole is based in large part on the value obtained for the phase component that is in phase with the transmitter current in the transmitter coil. This component signal is typically referred to as the real or "R" phase component. Measurement of a phase component signal which has a phase orthogonal to the transmitter current is sometimes obtained. This component signal is generally referred to as the "X" phase component signal.
Measurement of both the R and X phase component signals of the receiver signal is well known. U.S. Pat. Nos. 3,147,429 and 3,179,879 both disclose induction logging tools which detect phase quadrature components of the receiver signal from the receiver coil. The tools disclosed in these patents show the output from a receiver amplifier being applied to identical phase sensitive detector circuits, one for detecting the R component signal and the other for detecting the X component signal. Appropriate phase shifting components are provided for generating the phase quadrature phase reference signals required for the phase sensitive detectors, in order to resolve the phase component signals.
Numerous patents have been issued which disclose techniques for eliminating phase .shift errors which may be present in induction logging tools. These errors generally arise as static phase shift errors and dynamic or temperature dependent phase shift errors. Static phase shift errors are those phase shifts which occur when the tool is operated at a steady temperature condition. These phase shift errors are introduced into the detected phase component by certain electrical components within the tool. Dynamic phase shift errors occur as a result of the influences of temperature drift on the detection circuits.
U.S. Pat. No. 3,340,464 discloses a circuit for automatically adjusting the varying phase shifts which occur as a result of temperature drift by deriving a test signal from the current in the transmitter coil and thereafter substituting this test signal for the normal receiver coil output signal, generating a quadrature reference signal to the phase sensitive detector to detect a phase component within the receiver signal, and, phase shifting the reference signal as a function of the magnitude of the detected phase component signal in a direction to minimize that signal. This phase error compensation circuit does not attempt to segregate the relatively fixed or constant phase errors which arise within a tool from temperature dependent phase errors which vary with time during logging and resulting from component drift within the circuits. That is, this technique attempts to compensate for any and all phase shifts, regardless of the source of those phase shifts, which have occurred since the last phase compensation.
Another known phenomena in induction logging is the difference in the formation response as a function of frequency and formation conductivity. In general, the response signal received by an induction tool at low conductivities increases as the square of the frequency for a constant transmitter current. Because of the greater formation response at higher frequencies than at lower frequencies over most of the conductivities encountered, it becomes apparent that a low distortion transmitter signal is required. The more distorted a transmitter signal is the larger in amplitude are the harmonics of the fundamental frequency. Such harmonics propagate through the formation from the transmitter to the receiver with an attenuation and phase shift which are not related to those of the fundamental frequency. Thus, these effects may introduce false signals into the receiver that may cause a misleading result to be obtained from the induction tool measurement.
The variation in formation response with frequency may also be utilized beneficially to extend the range of formation resistivity that may be accurately measured by an induction logging tool. At high formation conductivities and higher frequencies a phenomena known as "skin effect" causes a loss of proportionality between the receive signal and the formation conductivity, introducing additional complexity in the interpretation of the signals.
Additionally, at lower transmitter frequencies and low conductivities, the response from the formation falls below the noise level of the induction logging system. In such cases, meaningful measurements are impossible. Thus, when encountering low conductivities, a high frequency for the transmitter signal would provide a more accurate reading of the formation conductivity. However, because of the sloping away of the response curves for the higher frequencies at higher conductivities, it would be desirable to have a lower transmitter frequency at high conductivities to avoid ambiguity in the conductivity derived from these measurements. This may be achieved by selection of a single frequency appropriate to the conductivity range expected prior to logging or by the generation of two or more frequencies simultaneously in the transmitter, with subsequent frequency separation in each receiver circuit and in each phase selective detection circuit or by sequentially switching to different frequencies while logging.
U.S. Pat. No. 4,449,421 discloses a digital induction logging system which includes means for generating a plurality of transmitter frequencies. In this manner, selection of the transmitter frequency may be based on optimizing the measurement of a characteristic of the formations being encountered by the tool. Automatic phase compensation is disclosed within U.S. Pat. No. 4,499,421, and is utilized to dynamically compensate for both static and dynamic temperature dependent phase errors due to circuits of the tool involved in the component measurements. A floating point analog-to-digital converter capable of responding to the wide dynamic range in the detected phase component signals is also provided within U.S. Pat. No. 4,499,421, in order to convert the phase detector output into digital signals for use by a processor.
While the aforementioned system provides an induction tool which permits the frequency of the transmitter signal to be selectable from among a plurality of transmitter signals those skilled in the art will appreciate that it would be advantageous to provide an induction logging system which is capable of simultaneously transmitting multiple frequencies during logging operations. Further, it would clearly be advantageous to provide a conversion circuit which is relatively insensitive to gain or phase variations within the amplifier or phase sensitive detector.